1. Field of the Invention
The present invention relates to a zoom lens system having a high zoom ratio, and more precisely, relates to a physically small zoom lens having a high zoom ratio more than 3, that can be advantageously used for a compact camera in which the back-focal distance is restricted.
2. Description of the Related Art
In a known zoom lens for a compact camera, having a high degree of magnification and a small length for the whole lens system, when set at a long focal length the focusing sensitivity (ratio of the displacement of the focal point to an error in displacement of the lens groups in the optical axis direction) at the long focal length is very large. This means that the focal point can deviate greatly from the film surface due to a small error in the displacement of the lens groups, thus resulting in a deterioration in the image quality. To resolve this problem, the present invention provides a zoom lens comprised of three lens groups including a first positive lens group, a second positive lens group, and a third negative lens group. The first and third lens groups are moved together (e.g., Japanese Un-examined Patent Publication No.Hei 2-256015). However, the variable power (magnification) of the zoom lens is approximately 2.5, and accordingly, there is still a need to increase the variable power.